This article reports on a research project aimed at determining the conditions for using sodium-hydroxide injection on the basis of a better understanding of the physicochemical mechanisms involved in the cil recovery process. The interfacial activity of sodium hydroxide is mainly linked to the chemical composition of the crude cils to be displaced, in particular to their acid content. Depending on this content, the lowering of interfacial tension in the presence of sodium hydroxide may be permanent or merely transitory. It also depends on the sodium-hydroxide concentration utilized and on the salinity of the injection water. Recovery tests mode in sandstone and limestone media with different crude oils show that the improvement in recovery obtained with sodium hydroxide is the result of the combined action of two mechanisms, i. e. a considerable reduction in the water/cil interfacial tension and an increase in the wafer wettability of the rock. The latter effect has been revealed by supplementary imbibition tests or by comparing relative permeabilities determined with and without thé additive in the aqueous phase.